1. Field of the Invention
The present invention relates to a multiple-tuned circuit and a probe for use in a nuclear magnetic resonance (NMR) spectrometer and, more particularly, to a multiple-tuned circuit and a probe used in an NMR spectrometer and which show enhanced resistance to RF voltages by performing a balanced operation.
2. Description of Related Art
In the description given below, a higher NMR frequency and a lower NMR frequency are often designated as HF (higher frequency) and LF (lower frequency), respectively. FIGS. 1 and 2 show the conventional double-tuned circuit. In FIG. 2, the amplitude of an RF voltage developed across the coaxial resonators 2, 3 when a higher frequency for an HF nucleus is at resonance are shown simultaneously. The circuit shown in FIGS. 1 and 2 can be tuned to the resonance frequency of an HF nucleus (e.g., 1H nucleus). In addition, the circuit can be simultaneously tuned to the resonance frequency of an LF nucleus (e.g., 13C nucleus). The circuit has a sample coil 1 consisting of a solenoid coil, saddle coil, or the like. Coaxial resonators 2 and 3 have a length equal to a quarter wavelength of the resonance frequency of the HF nucleus. The outer conductors of the resonators 2 and 3 are grounded during use. The coaxial resonator 2 that is electrically open is connected with one end of the sample coil 1, while the coaxial resonator 3 that is short-circuited is connected with the other end. A tuning variable capacitor 4 and a matching variable capacitor 5 are connected with the HF input/output side. A tuning variable capacitor 6 and a matching variable capacitor 7 are connected with the LF input/output side. A capacitor 8 acts to compensate for insufficiency of the capacitance of the LF tuning variable capacitor 4.
The operation is described next. As shown in FIG. 2, at the resonance frequency of the HF nucleus, the RF voltage developed across the open coaxial resonator 2 assumes a minimum amplitude of 0 at the upper end (as shown in the drawings) and a maximum amplitude of Vm at the lower end. The RF voltage at the shorted coaxial resonator 3 assumes a maximum amplitude of Vm at the upper end and a minimum amplitude of 0 at the lower end. The frequency can be adjusted with the tuning variable capacitor 4. Since the voltage amplitude is minimal at the upper end of the open coaxial resonator 2 at this time, HF power flowing into the LF side is small. At the resonance frequency of the LF nucleus, the open coaxial resonator 2 is not associated but the shorted coaxial resonator 3 acts as a grounded inductance L. Therefore, the frequency can be adjusted by the tuning variable capacitor 6 connected in parallel with the sample coil 1 and the opened coaxial resonator 2. In this way, this type of double-tuned circuit can adjust the HF and LF independently.
FIG. 3 shows another conventional double-tuned circuit. Note that like components are indicated by like reference numerals in various figures including FIGS. 1 and 2. A sample coil 1 consists of a solenoid coil, saddle coil, or the like. Two conductors 31 and 32 have a length equal to a quarter wavelength of the resonance frequency of an HF nucleus and form a parallel transmission line. The conductors 31 and 32 are grounded via tuning capacitors 10 and 11 for an LF nucleus during use. The sample coil 1 is connected between the two conductors 31 and 32. The conductors 31 and 32 are surrounded by a conductive outer tube 14 that is grounded. A tuning capacitor 9 for an HF nucleus is connected with the conductor 31. A tuning variable capacitor 4 and a matching variable capacitor 5 for the HF nucleus are connected with the conductor 32. At this time, the tuning variable capacitor 4 for the HF nucleus and the tuning capacitor 9 for the HF nucleus are so set up that their capacitances are nearly equal. A tuning variable capacitor 6 and a matching variable capacitor 7 for the LF nucleus are connected with the conductor 31.
At the resonance frequency of the HF nucleus, the tuning capacitors 10 and 11 for the LF nucleus have large capacitances and so their impedances are small. The conductors 31 and 32 are equivalent to the case where their ends are short-circuited. The conductors 31 and 32 are grounded together with the outer tube 14. As a result, the conductors 31, 32 and the outer tube 14 together operate as a quarter wavelength balanced resonant circuit at the resonance frequency of the HF nucleus. In particular, with respect to the conductors 31 and 32, the capacitance of the tuning variable capacitor 4 for the HF nucleus and the capacitance of the tuning capacitor 9 for the HF nucleus are set to nearly equal values. Therefore, RF voltages Vm/2 andxe2x88x92Vm/2 which are substantially equal in amplitude but opposite in polarity are produced at the opposite ends of the sample coil 1. Electrical currents of opposite polarities flow through the conductors 31 and 32 by the action of a kind of transformer. These RF voltages are halves of the voltage Vm shown in FIGS. 1 and 2. These voltages are applied to the tuning variable capacitors 4 and 5 for the HF nucleus.
Meanwhile, at the resonance frequency of the LF nucleus, the tuning capacitors 10 and 11 for the LF nucleus and the tuning variable capacitor 6 for the LF nucleus together form an LC resonant circuit. The capacitor 10 is connected in series with the sample coil 1 and the conductor 31. Similarly, the capacitor 11 is connected in series with the sample coil 1 and the conductor 32. The tuning variable capacitor 6 is connected in parallel with the capacitor 10. The frequency can be adjusted with the tuning variable capacitor for the LF nucleus. At this time, RF voltages which are almost equal in amplitude but opposite in polarity are produced at the opposite ends of the sample coil 1 by appropriately setting the capacitance of the tuning capacitors 10 and 11 for the LF nucleus. Therefore, the RF voltages applied to the tuning variable capacitors 6 and 7 for the LF nucleus can be held down to halves of the values in the case of FIGS. 1 and 2.
In the example of FIGS. 1 and 2, one end of the sample coil 1 is at ground potential at HF resonance and is near ground potential at LF resonance. Therefore, at HF resonance, a potential difference corresponding to the maximum amplitude at HF is directly applied across the variable capacitors 4 and 5. At LF resonance, a potential difference corresponding to the maximum amplitude at LF is directly applied across the variable capacitors 6, 7 and capacitor 8. Therefore, when high electric power is applied to the sample coil 1, electric discharging takes place, thus damaging these electrical parts.
The extraction line from the sample coil 1 is lengthened. This creates loss in the current path at LF resonance. Consequently, it is impossible to increase the resonance frequency of the LF nucleus. In this case, it may be conceivable to increase the resonance frequency by adding a dummy coil in parallel with the sample coil 1 to lower the inductance of the whole coil assembly. If this countermeasure is taken, however, an electrical current also flows through the dummy coil, increasing power loss. In this way, this countermeasure is inappropriate.
The configuration of FIG. 3 has the advantage that the voltage applied to the tuning variable capacitor 4 and matching variable capacitor 5 at HF resonance and the voltage applied to the tuning variable capacitor 6 and matching variable capacitor 7 at LF resonance are halves of the voltages applied in the case of FIG. 1. The conductors 31 and 32 are connected in series with the sample coil 1. Therefore, these conductors act as extraction lines at LF resonance. This increases the inductance of the whole coil assembly. As a result, the LF resonance frequency drops.
In the configuration of FIG. 3, it is necessary to connect capacitors 10 and 11 having considerably large capacitance in order to operate the conductors 31 and 32 as a quarter wavelength balanced resonator circuit for HF. If the capacitance of a capacitor is increased, the LF resonance frequency will drop concomitantly.
In view of the foregoing, it is an object of the present invention to provide a multiple-tuned circuit and a probe for use in a nuclear magnetic resonance spectrometer, which permit injection of high frequency electric power by improving the power capacity of the multiple-tuned circuit, and enhance resonance frequencies at both higher frequency (HF) and lower frequency (LF) resonances.
A multiple-tuned circuit for use in a nuclear magnetic resonance spectrometer in accordance with the present invention solves the problem described above and comprises: a sample coil having ends A and B; a first conductor having one end connected with the end A of the sample coil and another end connected to ground via a capacitive element or directly; a second conductor having one end connected with the end B of the sample coil and another end connected to ground via a capacitive element or directly; a tuning capacitive element for a second frequency, the tuning capacitive element being inserted in at least one of the junction (also referred to herein as the first junction) of the end A of the sample coil and the first conductor, a given position closer to the first conductor than the first junction, the junction (also referred to herein as the second junction) of the end B of the sample coil and the second conductor, and a given position closer to the second conductor than the second junction; a matching circuit and a tuning circuit for a first frequency; and a matching circuit for the second frequency.
In one feature of the multiple-tuned circuit described above, the first and second conductors and the capacitive elements together form a quarter wavelength resonator for the first frequency. Some of the capacitive elements are connected to the ends of the conductors. The other capacitive elements are connected with the given positions closer to the conductors than the ends.
In another feature of the multiple-tuned circuit described above, the matching circuit for the first frequency is located in a given position in the first conductor or in a given position on the second conductor.
In a further feature of the multiple-tuned circuit described above, the tuning circuit for the first frequency is located in at least one of a given position in the first conductor and a given position in the second conductor.
In still another feature of the multiple-tuned circuit described above, the matching circuit for the second frequency is located in a given position in the first conductor or in a given position in the second conductor.
In yet another feature of the multiple-tuned circuit described above, electric circuit components, such as capacitive elements and inductive elements, are replaceably added in parallel with the tuning capacitive element for the second frequency, the tuning capacitive element being inserted in at least one of the first junction of the end A of the sample coil and the first conductor, a given position closer to the first conductor than the first junction, the second junction of the end B of the sample coil and the second conductor, and a given position closer to the second conductor than the second junction.
In an additional feature of the multiple-tuned circuit described above, the first frequency is higher than the second frequency.
In yet another additional feature of the multiple-tuned circuit described above, there is further provided a third conductor. One end of this third conductor is connected with at least one of the ends of the first and second conductors. The other end of the third conductor is connected with a matching circuit for the third frequency and grounded via a capacitive element.
In a further feature of the multiple-tuned circuit described above, the first frequency is higher than the second and third frequencies.
The present invention also provides a probe for use in an NMR spectrometer, the probe including a multiple-tuned circuit comprising: a sample coil having ends A and B; a first conductor having one end connected with the end A of the sample coil and another end connected to ground via a capacitive element or directly; a second conductor having one end connected with the end B of the sample coil and another end connected to ground via a capacitive element or directly; a tuning capacitive element for a second frequency, the tuning capacitive element being inserted in at least one of the first junction of the end A of the sample coil and the first conductor, a given position closer to the first conductor than the first junction, the second junction of the end B of the sample coil and the second conductor, and a given position closer to the second conductor than the second junction; a matching circuit and a tuning circuit for a first frequency; and a matching circuit for the second frequency. This probe is characterized in that there is further provided a tubular electrode surrounding at least the outer surfaces of electrical circuit portions of the multiple-tuned circuit, and that this tubular electrode is used as a grounding electrode for the multiple-tuned circuit.
In one feature of this NMR probe, the first and second conductors and the capacitive elements together form a quarter wavelength resonator for the first frequency. Some of the capacitive elements are connected to the ends of the conductors. The other capacitive elements are connected with the given positions closer to the conductors than the ends.
In another feature of the NMR probe described above, the matching circuit for the first frequency is located in a given position in the first conductor or in a given position in the second conductor.
In a further feature of the NMR probe described above, the tuning circuit for the first frequency is located in at least one of a given position in the first conductor and a given position in the second conductor.
In still another feature of the NMR probe described above, the matching circuit for the second frequency is located in a given position in the first conductor or in a given position in the second conductor.
In yet another feature of the NMR probe described above, electric circuit components, such as capacitive elements and inductive elements, are replaceably added in parallel with the tuning capacitive element for the second frequency, the tuning capacitive element being inserted in at least one of the first junction of the end A of the sample coil and the first conductor, a given position closer to the first conductor than the first junction, the second junction of the end B of the sample coil and the second conductor, and a given position closer to the second conductor than the second junction.
In an additional feature of the NMR probe described above, the first frequency is higher than the second frequency.
In yet another additional feature of the NMR probe described above, there is further provided a third conductor whose one end is connected with at least one of the ends of the first and second conductors, the other end being connected with a matching circuit for the third frequency and grounded via a capacitive element.
In a further feature of the NMR probe described above, the first frequency is higher than the second and third frequencies.
In still another additional feature of the NMR probe described above, the aforementioned tubular electrode is provided with an opening in a given position to place the inside of the probe in communication with the outside.
Other objects and features of the invention will appear in the course of the description thereof, which follows.